1. Field of the Invention
The present invention relates to heat developable photosensitive materials. Particularly, it relates to heat developable photosensitive materials having less heat fog and good whiteness.
2. Description of the Prior Art
The photographic process using silver halides has been most widely practiced hitherto, because excellent photographic properties such as good sensitivity or gradation can be obtained in this process as compared with other photographic processes such as an electrophotographic process or a diazo photographic process. However, silver halide photosensitive materials used in this process are subjected to development using a developer after image exposure and then subjected to processings such as stopping, fixation, water washing or stabilization so as to prevent the developed images from fading or discoloration under normal room illumination to prevent the undeveloped areas (hereinafter called background) from blackening. Accordingly, these processings take much time and are labor some. Further, there are problems in that the handling of the chemicals used is dangerous to the human body or the hands and clothes of the workers and the processing room are stained at processing. Thus, it is very desired to improve the photographic process using silver halides so that the processings can be carried out in a dry manner without using solution processing and the processed images can be preserved in a stabilized state.
Therefore, many approaches to achieve such have been attempted hitherto. One approach is to use heat developable photosensitive materials as described in U.S. Pat. Nos. 3,152,904, 3,457,075, 3,635,719, 3,645,739, and 3,756,829, Canadian Patent 811,677. This approach is to use a photosensitive element wherein silver salts, for example, silver salts of higher fatty carboxylic acids such as silver behenate, silver saccharin or silver benzotriazole are used as a main component and a catalytic amount of silver halide is used.
However, in these heat developable photosensitive materials, the quality of the resulting image is not good because of heat fog, namely, undesirable fog occurs to a very high degree when the nonexposed area is heated. Further, undesirable residual color of color compounds, for example, residual dyestuffs adsorbed in silver halide occur. This residual color injures the quality of the resulting image.
As a technique of inhibiting heat fog, it has been said that mercury compounds are effective, as described in, for example, U.S. Pat. No. 3,589,903. However, as is well known, mercury compounds are toxic, and the use of sensitive materials containing such a toxic material is not preferred for health and safety reasons. Even though the toxicity is low itself, indirectly a serious problem occurs in the production of regenerated paper if paper is used as a support for such an element.
No technique of improving the whiteness (or removing residual color) has been known hitherto.